1. Technical Field
The present invention relates to an apparatus for contactlessly measuring a measurement object, more precisely the surface of a measurement object, with an optical sensor.
2. Prior Art
Various embodiments of such measurement apparatuses for a contactless measurement of measurement objects are already disclosed in the prior art, contactless measurements often being combined with contact-making, i.e. mechanically probing, measurements.
Furthermore, such measurement apparatuses are known both for one-dimensional measurement only in one coordinate axis (z axis) and for multicoordinate measurement in the form of 2D or 3D measurement apparatuses in which, in addition to the z coordinate of the measurement point on the measurement object, the x and/or y coordinate thereof can also be detected.
Such a multicoordinate measurement apparatus is disclosed for example in U.S. Pat. No. 4,908,951 by W. H. Gurny. In the case of the apparatus disclosed in this document, both a contactless and a mechanically probing measurement of a measurement object are possible by means of a multisensor probing system. In this case, the measurement center sleeves of the optical sensor for the contactless measurement and of the mechanical probe head for the mechanically probing measurement are mounted on a common or on two separate slides which can be moved in the x and y coordinate directions. For automatic measurement-object contour detection, the optical sensor has a laser sensor.
The laser sensor disclosed in U.S. Pat. No. 4,908,951 follows the surface contour of the measurement object at a constant distance and thus achieves a contour detection in real time with high accuracy and a high scanning speed. This measurement principle is based on a so-called split-beam method, in which the reflective surface of the measurement object is used as a reference for focussing. The laser sensor directs a light beam via an objective onto the measurement object's surface to be measured, from which the light beam is reflected and guided via an optical arrangement onto a receiver device equipped with a plurality of differential diodes. On account of the imaging method employed in this system, the signal received by the receiver device drifts in the event of defocussing of the light beam on the surface of the measurement object and generates a differential signal which is fed to an advancing apparatus in the form of a servomotor. The advancing apparatus positions the optical sensor in accordance with the differential signal in the z coordinate direction in such a way that the measurement object's surface to be measured again lies in the focal plane of the objective. The position of the optical sensor in the z coordinate direction is detected by means of a suitable measurement system and fed to an evaluation unit.
On account of the continuous and automatic focussing of the measurement object, high measurement accuracies in conjunction with high scan speeds can be achieved with a measurement apparatus of this type. However, in this system, the z coordinate of a measurement point determined on the measurement object is affected by a so-called contouring error. This contouring error arises because the mechanical readjustment of the position of the optical sensor in the z coordinate direction is not exactly contemporaneous with the reading command of the receiver device, i.e. the receiver device reads out a measurement value for the z coordinate which does not precisely correspond to an exact focussing of the measurement object. This error is dependent, in particular, on the resolution of the measurement system, the scan speed and the geometrical errors in the z axis.